(a) Field of the Invention
The present disclosure relates to an olefin-based polymer with excellent processability.
The present application is based on, and claims priority from Korean Patent Application Nos. 10-2013-0139997 and 10-2014-0114385, filed on Nov. 18, 2013 and Aug. 29, 2014, the disclosure of which is hereby incorporated by reference herein in its entirety.
(b) Description of the Related Art
Olefin polymerization catalyst systems may be divided into Ziegler-Natta and metallocene catalyst systems, and these two highly active catalyst systems have been developed in accordance with their characteristics. The Ziegler-Natta catalyst has been widely applied to commercial processes since it was developed in the 1950's. However, since the Ziegler-Natta catalyst is a multi site catalyst with several different active sites, it is characterized by producing polymers with a broad molecular weight distribution. Also, since compositional distribution of comonomers is not uniform, it is difficult to provide the desired physical properties.
Meanwhile, the metallocene catalyst includes a main catalyst whose main component is a transition metal compound and an organometallic compound cocatalyst whose main component is aluminum. Such a catalyst is a homogeneous complex catalyst and a single-site catalyst, and offers a polymer having a narrow molecular weight distribution and uniform compositional distribution of comonomers, depending on the single site characteristics. The stereoregularity, copolymerization characteristics, molecular weight, crystallinity, and so forth of the polymer may be controlled by changing a ligand structure of the catalyst and polymerization conditions.
U.S. Pat. No. 5,914,289 describes a method of controlling the molecular weight and the molecular weight distribution of polymers using metallocene catalysts which are respectively supported on supports. A large amount of solvent and a long preparation time are required for preparing the supported catalysts, and the process of supporting metallocene catalysts on the respective supports is troublesome.
Korean Patent Application No. 2003-12308 discloses a method of controlling the molecular weight distribution by polymerizing polymers while changing a combination of catalysts in a reactor by supporting a dinuclear metallocene catalyst and a mononuclear metallocene catalyst on a support together with an activating agent. However, this method is limited in simultaneous implementation of properties of the respective catalysts. In addition, there is a disadvantage that a metallocene catalyst portion is departed from a final supported catalyst to cause fouling in the reactor.
Therefore, to solve the above drawbacks, there is a continuous demand for a method of preparing olefin-based polymers with the desired physical properties by easily preparing a hybrid supported metallocene catalyst with an excellent activity.
Meanwhile, linear low-density polyethylene is prepared by copolymerization of ethylene and alpha olefin at a low pressure using a polymerization catalyst, and is a resin having a narrow molecular weight distribution and short chain branches with a predetermined length, along with the lack of long chain branches. Linear low-density polyethylene films have high strength at break and elongation, and excellent tear strength and falling weight impact strength, in addition to general characteristics of polyethylene, and therefore, their use is growing in the fields of stretch films, overwrap films, etc., to which the existing low-density polyethylene or high-density polyethylene has been hardly applied.
A preparation process of linear low-density polyethylene using 1-butene or 1-hexene as a comonomer is generally performed in a single gas phase reactor or a single loop slurry reactor, and its productivity is higher than a process using 1-octene comonomers. However, due to limitations of catalyst and process technologies, the product has physical properties inferior to those of a product obtained by using 1-octene comonomers, and has a narrow molecular weight distribution to show poor processability. Many efforts have been made to improve these problems.
U.S. Pat. No. 4,935,474 discloses a method of preparing polyethylene with a broad molecular weight distribution by using two or more metallocene compounds. U.S. Pat. No. 6,828,394 discloses a method of preparing polyethylene having excellent processability, and in particular, being suitable for films by using a mixture of a “good comonomer incorporator” and a “poor comonomer incorporator”. In addition, U.S. Pat. Nos. 6,841,631 and 6,894,128 disclose that polyethylene having a bimodal or multimodal molecular weight distribution is prepared by using a metallocene catalyst containing at least two metal compounds, thereby being applied to films, blow molded products, pipes, etc. However, even though these products have improved processability, the dispersion state according to the molecular weight in a unit particle is not uniform, and thus appearance is rough and physical properties are not stable under relatively desirable extrusion conditions.
Under this background, there is a continuous demand for an excellent product in which a balance between physical properties and processability is ensured, and improvement thereof is further required.